1. Field of the Invention
This invention relates to a tanning composition and tanning method and more particularly to a pretanning composition which is useful in increasing the shrinkage temperature of a hide.
2. Description of the Prior Art
The tanning of hides to prepare leather has been known for many years. The tanning involves a tanning agent which reacts with the hide to prevent putrefaction and degradation thereof.
Over the years it has been found that for nearly all leathers, chrome is a preferred tanning agent. The wastes from chrome tanning processes, however, contain pollutants particularly chrome which is undesirable for enviromental reasons. These pollutants are introduced during the disposal of shavings and leachings from dumped shavings and in the disposal of chrome tanning liquors.
As is described in U.S. Pat. No. 4,060,384, incorporated herein by reference, the amount of chromium used in chrome tanning can be reduced by various techniques such as conducting a pretanning operation with tanning agents that are essentially free of chromium.
Glutaraldehyde has been suggested for pretanning hides that are subsequently chrome tanned, but is considered to lower the tensile and tear strengths of the final leather. Further, the reaction product of the glutaraldehyde with the functional groups, particularly amino groups, of the protein constituents of the leather produce chromophores which impart an undesirable color to the final leather product. Further, the odor of glutaraldehyde is objectionable. An example of such glutaraldehyde tanning is reported in the paper entitled "Oxazolidines--A new Class of Tanning Agent", by S. Dasgupta, published in the Journal of the Society of Leather Technologists and Chemists, Vol. 61, pp. 97-105 (1977). The reductions in tensile are experienced even though the chrome content of the finished leather is higher than for a straight chrome tanning.
Further examples of more efficient chrome tanning are shown in U.S. Pat. No. 3,888,625 wherein sulfide salts are used in conjunction with an aldehyde such as formaldehyde, glutaraldehyde or benzaldehyde.
U.S. Pat. No. 4,060,384 suggests pretanning with mixtures containing glutaraldehyde and other aldehydes, however, the compounds of the present invention are more effective than those disclosed in U.S. Pat. No. 4,060,384.
A pretanning composition which is effective in increasing the shrinkage temperature of a hide along with a decrease in the amount of chrome required for tanning is disclosed in U.S. patent application Ser. No. 052,149 of Marcel Siegler filed the same day as this application. Said U.S. patent application is incorporated herein by reference. Said patent application discloses the reaction product of a polyaldehydic compound and a secondary amine to form a pretanning composition. The resulting reaction product is believed to be represented by the structural formula: ##STR1##
wherein Y is a polyvalent hydrocarbon residue; R.sub.1 and R.sub.2 are alkylene and may be the same or different and X.sub.1 and X.sub.2 are hydrogen or OH and may be the same or different; "a" is 0 or 1; and Z is greater than 1.
The compounds according to the said patent application are somewhat unstable and on aging tend to alter in structure and precipitate from aqueous solution. The mechanism which imparts this instability is believed to be attributable to the hydroxyl group which is .alpha. to the amino nitrogen. These .alpha. hydroxyl groups may condense and hence polymerize to form acetyl type polycondensates.
In accordance with the present invention the pretanning composition disclosed in U.S. patent application Ser. No. 052,149 of Marcel Siegler filed the same date as this application is stabilized.